Cable comprising optical fibres enclosed in an outer sheath

ABSTRACT

The cable includes optical fibers (1) enclosed in an outer sheath (2) having elongate reinforcing members (4) embedded therein, disposed in two diametrically opposite groups (3) each comprising a plurality of mutually tangential reinforcing members (4).

The present invention relates to a cable including optical fibersenclosed in an outer sheath.

BACKGROUND OF THE INVENTION

In the cable industry, it is well known that optical fibers must notsuffer mechanical stress if both their transmission performance andtheir lifetime are not to be degraded.

To limit traction stresses on optical fibers while cables are beinglaid, or during temperature variations which cause expansion orretraction of the optical fibers, it is known to use reinforcing memberswhich may be central or peripheral. In the case of a central reinforcingmember it is generally necessary to twist the optical fibers or theelements containing the optical fibers. This slows down the rate ofproduction and increases the cost of the cable obtained.

To avoid the twisting operation, it is known to make an optical fibercable by enclosing a bundle of optical fibers in an outer sheath havingreinforcing members embedded longitudinally therein. When a cable ofthis type is subjected to bending, the reinforcing members which arelocated on the outside of the bend relative to the middle of the cable,are lengthened by an amount that increases with increasing diameter ofthe cable. This results in resistance of the cable to bending and in therisk of the reinforcing members breaking if the force applied to thecable is greater than a force which causes the outermost reinforcingmembers to lengthen to breaking point.

To avoid that problem, it has been envisaged in document DE-A-38 15565to dispose only two diametrally opposite reinforcing members, or even todispose diametrally opposite reinforcing members in groups includingthree spaced-apart reinforcing members or four reinforcing membersarranged in a housing filled with a material having a low coefficient offriction to enable the reinforcing members to move relative to thesheath. However, in the case of spaced-apart reinforcing members it isnot possible to bend the cable about an axis parallel to the lineinterconnecting the reinforcing members without subjecting thereinforcing members to differential elongation which risks causing themto break. Furthermore, in the case of reinforcing members disposed in ahousing, the outer sheath does not participate in the traction force andit is thus necessary to provide a greater number of reinforcing members.In both cases the sheath in which the reinforcing members are embeddedmust itself be relatively thick, thereby reducing the bending capacityand reduces the compactness of the cable.

OBJECTS AND SUMMARY OF THE INVENTION

The invention provides a cable including optical fibers enclosed in anouter sheath having elongate reinforcing members embedded therein,disposed in two diametrally opposite groups, each comprising a pluralityof adjacent reinforcing members, wherein the reinforcing members in eachgroup are mutually tangential.

Thus, in allowing the cable to bend about an axis parallel to a lineinterconnecting the two groups of reinforcing members, it is possible toobtain a total cross-section of reinforcing members in which thediameter of each reinforcing member is reduced and, consequently, thethickness of the outer sheath is reduced. The bending capacity of thecable is thus increased.

Furthermore, the distance of the reinforcing members from the middle ofthe cable is minimized, and the differential elongation between thereinforcing members is consequently minimized, thereby allowing greaterbending.

According to an advantageous aspect of the invention, the number ofreinforcing members of each group is selected so that when the cablebends around the minimum radius of curvature of the cable about an axisparallel to a line interconnecting the two groups of reinforcingmembers, the reinforcing member subjected to the greatest stress iselongated by an amount that is less than the breaking elongation of saidreinforcing member. There is thus an optimum compromise between thenumber of reinforcing members that allow the thickness of the outersheath to be minimized and the stresses to which the reinforcing membersare subjected.

BRIEF DESCRIPTION OF THE DRAWING

Other characteristics and advantages of the invention appear on readingthe following description of a particular non-limiting embodiment of theinvention, given with reference to the accompanying sole FIGURE, whichis a perspective view of the end of a cable of the invention in a bentposition.

MORE DETAILED DESCRIPTION

In FIG. 1, the cable of the invention includes a bundle of opticalfibers 1, the structure of which is not shown in detail, but which may,for example, be made in the form of one or more groups of optical fibersheld together by holding sheaths as described in document FR-A-2 665266. The bundle of optical fibers is enclosed in an outer sheath 2having two groups 3 of reinforcing members 4 embedded therein. Thegroups 3 are diametrally opposite and each comprises two mutuallytangential reinforcing members 4 disposed either side of a line Dpassing via the middle of the bundle of optical fibers. The reinforcingmembers are made of conventional materials such as metal wires, orstrands of glass fiber or aramid, stiffened by a resin having a hightraction modulus and a low coefficient of thermal expansion.

By disposing two reinforcing members side by side in each group, it canbe seen that for the same total reinforcing member cross-section aswould be required using only two diametrally opposite reinforcingmembers, it is possible to use reinforcing members having a diameterthat is divided by 1.4 relative to the diameter that would be necessarywith only two diametrally opposite reinforcing members. The thickness ofthe outer sheath is thus considerably reduced compared with thethickness of an outer sheath in which only two reinforcing members areembedded.

In addition, if r designates the average radius of curvature of thecable about an axis Δ parallel to the line D, if r_(i) designates theaverage radius of curvature of the reinforcing member 4 that is radiallyinside the line D, and if re designates the average radius of curvatureof the reinforcing member that is radially outside the line D, and ifthe reinforcing members all have a diameter equal to half the thicknessof the outer sheath 2, then:

    r.sub.i =r-1/4e

and

    r.sub.e =r+1/4e

from which the differential elongation can be deduced as follows:

    (r.sub.e -r.sub.i)/r=1/2e/r

By way of example, differential elongation of only 1.2% is obtained whenr=50 mm and e=1.2 mm, whereas it is generally accepted that breakingelongation is about 5%.

By arranging reinforcing members in diametrally opposite groups ofadjacent members, the stresses on the diametrally opposite reinforcingmembers are not unduly increased, whereas the thickness of the outersheath is decreased substantially.

The number of reinforcing members of each group could be increasedwithout going beyond the ambit of the invention.

It can be seen, however, that in that case, the outermost fiber furthestfrom the line D is subjected to greater stress than are the other fibersand it is thus necessary to ensure that, for a minimum radius ofcurvature of the cable, the reinforcing member subjected to the greateststress is elongated by an amount that is less than its breakingelongation.

Naturally, the invention is not limited to the embodiment described andvariants thereof come within the ambit of the invention as defined bythe claims.

I claim:
 1. A cable including optical fibers (1) enclosed in an outer sheath consisting in a flexible layer (2) in which elongated reinforcing members (4) are disposed in two diametrically opposite groups (3) each comprising two reinforcing members (4) embedded in said flexible layer and distant from surfaces thereof, said two reinforcing members of each group being tangential to each other. 